An L‐Lactate Amperometric Enzyme Electrode Based on L‐Lactate Oxidase Immobilized in a Laponite Gel on a Glassy Carbon Electrode. Application to Dairy Products and Red Wine

A biosensor based on the immobilization of Lactate oxidase in laponite–organosilasesquioxane films on glassy carbon electrode for the quantification of L ‐lactate in wine and dairy products is presented. The bioelectrode showed a very high sensitivity (0.33±0.01) A cm^?2 M^?1 and a short time response (10 s) for less than 1 U of enzyme. No significant interferences, including ascorbic acid, were detected. For red wine, matrix effects assigned to polyphenols and anthocyanins were observed, which ware easily overcome by sample dilution. Our L ‐lactate determinations were in good agreement with those of two standard methods.     

The molecular symmetry and electronic spectroscopy of 7-azaindole dimer: its proton-transfer channels

The potential-energy surfaces for the proton transfer in the doubly hydrogen-bonded dimer of 7-azaindole in its lowest excited electronic states were examined. The dimer with C2h symmetry in its lowest excited electronic states, 2Ag and 1Bu, undergoes concerted double-proton transfer via transition states of the same symmetry placed at energies 4.55 and 4.70 kcal/mol higher, respectively. This suggests that the activation barriers for the double-proton transfer, if any, are lower than 1 kcal/mol. Emission from the dimers resulting from the double-proton transfer involves a Stokes shift of 5605 cm(-1), as theoretically estimated from the 0-0 components of the absortion and emission transitions of the dimer. Surprisingly, however, the calculations suggest that the green emission cannot arise from the 2Ag state generated by a double-proton transfer, because this structure possesses an imaginary frequency. In the 7-azaindole dimer of Cs symmetry, the first excited electronic state, a', lies 4.9 kcal/mol below 1Bu. This excited state a' can be the starting point for single-proton transfers giving a zwitterionic form that can dissociate into the protonated and deprotonated forms of 7-azaindole, the former being electronically excited. This situation of lower symmetry is consistent with the mutational scheme proposed by Goodman [Nature (London) 378, 237 (1995)].     

Kinetics of the hydrolysis of bisphenol A diglycidyl ether (BADGE) in water-based food simulants

Summary The first-order degradation kinetics of bisphenol A diglycidyl ether (BADGE; CAS No. 1675-54-3) has been studied in three water-based food simulants (3% (W/V) acetic acid, distilled water and 15% (V/V) ethanol) at various temperatures. BADGE and its first and second hydrolysis products were determined by reversed-phase high-performance liquid chromatography with fluorescence detection. Nonlinear regression was used to fit the experimental data at 40, 50 and 60°C with the proposed kinetic equations; Arrhenius' equation was then fitted to the rate constants obtained and the kinetic models were tested by comparing experimental data obtained at 70°C with the kinetic curves calculated using the rate constants predicted for this temperature. The half-life of BADGE was longest in ethanol and shortest in acetic acid. The rings opening in acetic acid appears to happen by means of active hydrogens whereas in the other simulants it is mainly influenced by the formation of acid/base adducts. The results imply that resins which comply with existing legislation on the migration of unreacted monomer may still contaminate foodstuffs.     

Kinetic studies on the formation of N-nitroso compounds

Acetic acid/acetate ion buffer acts catalytically upon the nitrosation of amines under conditions in which the only nitrosating agents are N₂O₃ and NOBr, but inhibits nitrosation by H₂NO ₂ ⁺ . The kinetic characteristics of these phenomena have been analysed quantitatively and compared with similar effects caused by the solventsTHF, DMSO and dioxane. The experimental results show that this behaviour is an effect of the medium.

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Kinetische Untersuchungen zur Bildung von N-Nitroso-Verbindungen, 8. Mitt.: Nachweis eines Medium-Effekts von Essigsäure/Acetat-Puffer auf die Geschwindigkeitskonstante der Nitrosierung

Zusammenfassung Essigsäure/Acetat-Puffer wirkt bei der Nitrosierung von Aminen katalytisch, unter Bedingungen, wo die alleinigen nitrosierenden Agentien N₂O₃ und NOBr sind; andererseits wird die Nitrosierung durch H₂NO ₂ ⁺ unterbunden. Die kinetischen Charakteristika dieses Phänomens wurden quantitativ analysiert und mit ähnlichen Effekten der LösungsmittelTHF, DMSO und Dioxan verglichen. Die experimentellen Ergebnisse zeigen, daß dieses Verhalten auf einen Mediumeffekt zurückzuführen ist.

     

Zeolite synthesis by simulation of their natural formation conditions: from macroscopic to nanosized crystals

A successful route to prepare zeolites with controlled crystal size from the millimeter range down to nanometer scale by simulating the natural formation conditions is presented. Water is used as the solution and reaction medium at high temperatures and pressures (T=120-400°C, p=1 kbar); aluminosilicate glasses with identical chemical composition as the zeolite products are used as precursors. The synthesis of heulandite is reported as representative example.     

Synthesis of heparin-like oligosaccharides on a soluble polymer support

Based on previously developed solution phase chemistry, an effective general approach to the synthesis of heparin-like oligosaccharides on a soluble polymer support is reported.     

PROMOTIONAL EFFECT OF Sn ON NO OXIDATION OVER Pt/Al2O3 AS PROMISING GAS EXHAUST CATALYTIC CONVERTER. SELECTIVE Pt AND Sn DISPERSION DETERMINATION

NO oxidation was investigated over Pt-Sn/g-Al₂O₃. The addition of Sn to 1%Pt/g-Al₂O₃ enhances NO oxidation to NO₂. This effect is related to the enhancement of chemisorbed oxygen on the metallic surface produced by Sn.     

The tautomerism of 1,2,3-triazole, 3(5)-methylpyrazole and their cations

The annular tautomerism of 1,2,3-triazole and 3(5)-methylpyrazole is discussed by means of a combination of theoretical calculations and experimental (ICR) gas-phase basicities and acidities. In the gas phase 1,2,3-triazole exists as the 2H-tautomer, whereas both tautomers of 3(5)-methylpyrazole are of similar energy. The solvent effects on these prototropic equilibria are discussed taking into account solvent properties as polarity/polarizability, acidity, and basicity. In nonhydrogen bonding solvents, the difference in dipole moments between both tautomers plays a role that has usually been underestimated.     

Experimental excess molar volumes of the ternary mixture and comparisonwith several empirical methods

Experimental excess molar volumes for the ternary system {x₁MTBE+x₂1-propanol+(1–x₁–x₂)nonane} and the three involved binary mixtures have been determined at 298.15 K and atmospheric pressure. Excess molar volumes were determined from the densities of the pure liquids and mixtures, using a DMA 4500 Anton Paar densimeter. The ternary mixture shows maximum values around the binary mixture MTBE+nonane and minimum values for the mixture MTBE+propanol. The ternary contribution to the excess molar volume is negative, with the exception of a range located around the rich compositions of 1-propanol. Several empirical equations predicting ternary mixture properties from experimental binary mixtures have been applied.     

Test study on the excitation spectra of the N<Subscript>2</Subscript>–He van der Waals molecule

We have performed ab initio fourth-order Møller–Plesset perturbation theory calculations in the framework of the supermolecule approach on the vertical excitation spectra of the weakly bound van der Waals N₂–He dimer. They indicate a ``T-shaped' stablest ground N<sub>2</sub>(X<sup>1</sup><sub>g</sub><sup>+</sup>)–He(<sup>1</sup>S) electronic state with a well depth, D<sub>e</sub>, of 21.63 cm<sup>–1</sup> at a minimum distance, R<sub>e</sub>, of 3.44 Å and zero-point vibration correction, D<sub>o</sub>, of 7.07 cm<sup>–1</sup>. They also indicate a ``T-shaped' stablest excited conformer with R_e=3.25 Å, D_e=36.85 cm^–1 and D_o=17.06 cm^–1 for the N₂(B³_g)–He(¹S) triplet electronic level. In order to investigate the use of less-demanding correlation methods, test density functional theory calculations using the mPW1PW exchange–correlation functional are also presented for comparison.